In his victory speech, Markey promised to go to Washington to fight for Massachusetts voters and, as President Obama readies a major push for climate change legislation, promised to lead on that issue.

“I want to lead the effort to launch a clean energy revolution in our country,” he said.

Gomez said he had faced an "overpowering force" in Markey and other Democratic groups, which significantly outspent Gomez, but added that he would "offer absolutely no excuse for coming up short."

“I mean, we were massively overspent. We went up against literally the whole national Democratic party and all its allies," he said.

“Today marks the end of the first mile in the marathon to permanently fill the Massachusetts Senate seat,” Moran said in a statement. “Gomez is well-prepared to win that marathon over the next 16 months.”

Markey will be up for election to a full term in 2014, and Gomez is rumored to be a potential candidate for that race, or another statewide office, potentially lieutenant governor.

On Tuesday, officials predicted record-low turnout, due to the odd timing of the race, the sweltering weather in the state and overall voter fatigue.

Markey's win comes as little surprise in the deep blue state, where Democrats outnumber Republicans 3-to-1, and President Obama won with more than 60 percent support in 2012.

But it follows a concerted campaign by Democrats to avoid the mistakes that cost them a seat in 2010, when former Sen. Scott Brown (R) orchestrated a surprise upset over Democrat Martha Coakley.

As recently as Friday, Massachusetts Democrats were expressing caution about their chances in the race.

“In Massachusetts, we do win more elections than the bad guys, but there’s a lot of them that are close. It's not a gimme,” Massachusetts Democratic Party Chairman John Walsh told The Hill on Friday.

But polling throughout the campaign had shown Markey with a consistent lead, ranging from mid-single digits to 20 percentage points.

The most recent poll of the race, issued Monday, gave Markey a 10-point lead over Gomez.

To hedge their bets, however, Democrats sent nearly every party leader available to the state, including Vice President Biden, former President Clinton and President Obama.

Democrats also outspent Gomez 3-to-1 in the race, and Markey received help from a bevy of outside groups, including the League of Conservation Voters, Senate Majority PAC and a group backed by California billionaire and environmental activist Tom Steyer.

The Democratic Senatorial Campaign Committee also boosted his campaign with staff and financial support.

In comparison, Gomez ran a lone wolf campaign, without much support from GOP groups or big party names. Only one group, which characterized itself as moderate, went on air for Gomez, and it did so in the later stages of the race.

The National Republican Senatorial Committee did come to the candidate's aid, spending nearly $1 million on the race and sending staff to help Gomez, but their efforts weren't enough to match those of Democrats.

Gomez, a young Latino former Navy SEAL with a career in private equity, was seen by many as exactly the kind of candidate the GOP needs to expand its appeal for future elections, and the lack of support from donors and big-money groups frustrated some Gomez supporters, who saw it as hypocritical.

But the candidate faced the difficulty unique to Republicans running in Massachusetts: He sought to run from his unpopular party, rather than with it.

To win in Massachusetts, he’d have to draw the support of a significant number of independents — from 55 to 60 percent, by Republican predictions — as well as some disaffected Democrats.

In search of those voters, Gomez characterized himself as a "new kind of Republican," and appearing alongside or receiving help from any establishment Republicans would have undermined that narrative.

But his efforts to campaign as an independent Republican fell short. Despite moderate positions on gun control — he supported the Toomey-Manchin background checks bill — and abortion rights — personally pro-life, Gomez said he wouldn’t go to Washington to change any abortion-rights laws on the books — Democrats successfully blasted him for opposing an assault weapons ban and his openness in supporting a pro-life Supreme Court nominee.

And they neutralized his business background by increasing pressure on Gomez to release a list of his clients during his time in private equity.

Republicans hit back with attacks on Markey’s near 40-year tenure in Washington and questions about where he resides and his voting record in Congress.

At times, the race became nasty, with Gomez comparing Markey to “pond scum” for Web ads his campaign ran in which a shot of Gomez’s face appeared alongside Osama bin Laden’s for a brief moment.

Ultimately, the partisan lean of the state trumped campaign attacks.

There's likely now to be fierce competition in the Democratic primary for the opportunity to take Markey's seat, which is reliably blue.